Genetic and demographic consequences of human-driven landscape changes on bird populations: The case of Aphrastura spinicauda (Furnariidae) in the temperate rainforest of South America

Human activities, such as expansion of agricultural land and forest exploitation, have modified landscapes worldwide. Despite a large accumulation of empirical and theoretical knowledge on habitat loss and fragmentation, some aspects remain poorly understood, especially those related to the interaction between different forms of habitat degradation. The overall goal of this research was to study the effects of habitat fragmentation and changes in forest structure on bird populations in a human-modified landscape in the temperate rainforest of South America using Aphrastura spinicauda (Furnariidae) as a model species. Specifically, I evaluated (i) if replacement of forest by open habitat leads to reduced dispersal influencing the genetic structure of populations (ii) if density and reproductive success vary across a fragmented landscape and if populations are limited because of nest-site availability in forests that differed in structure (modified by selective logging) and degree of isolation and (iii) if nest-site selection patterns and associated consequences on fitness are spatially variable in response to changes in forest structure and connectivity. First I show that forest replacement by open habitat reduced landscape connectivity and influenced the genetic structure of populations even within the time-space scale of habitat fragmentation caused by human activities. Secondly, I show that bird density varied across the fragmented landscape while nesting success remained constant. Aphrastura spinicauda is a non-excavator cavity-nester and thus relies on old trees or snags where most cavities form. A nest-site supplementation experiment revealed that nest-site limitation is the primary cause of density reduction in selectively-logged forests, but that other processes directly related to forest fragmentation (e.g. loss of connectivity) also influence population responses to habitat changes. Finally, I showed evidence for adaptive nest-site preferences, but also that nest-site choice is spatially variable in response to ecological gradients produced by human activities. By combining genetic and demographic responses of populations I provided evidence for unforeseen and potentially synergistic interactions among different forms of habitat degradation. These results emphasize the need of explicit and independent consideration of habitat fragmentation and other forms of habitat degradation, such as selective logging, when studying populations in human-modified landscapes. In that way, we can better understand and predict population persistence and their adaptive responses in these landscapes.